1. Field of the Invention:
The present invention relates to a process for producing a III-V group compound semiconductor light emitting device, particularly a nitrogen-containing III-V group compound semiconductor light-emitting device having nitrogen-containing III-V group compound semiconductor layer.
2. Description of the Related Art:
Gallium nitride (GaN) and aluminum nitride (AlN) have large band gaps and their band gap energies correspond to an ultraviolet light. Alloyed crystals between (1) gallium nitride and/or aluminum nitride and (2) indium nitride (InN), i.e. indium gallium nitride (InGaN), indium aluminum nitride (InAlN) and indium gallium aluminum nitride (InGaAlN) also have band gaps corresponding to a blue light or an ultraviolet light, depending upon their compositions. Therefore, these nitrogen-containing III-V group compound semiconductors (hereinafter each referred to as "nitride semiconductor") are drawing attention as a material for light (blue light or ultraviolet light)-emitting device, or as a material for high voltage-resistant or high temperature-resistant environmental electronic device. Nakamura et al. report, in Jpn. J. Appl. Phys. Vol. 35 (1996), p. L74, production of blue light laser device using the above compound semiconductors. FIG. 3 shows a sectional view of this laser device. As shown in FIG. 3, the laser device by Nakamura et al. has a structure obtained by forming, on a sapphire substrate 301 having a c-face [(0001) face] on the surface, a gallium nitride buffer layer 102 of 30 nm in thickness, a Si-doped n type gallium nitride layer 103 of 3 .mu.m in thickness, a Si-doped n type In.sub.0.1 Ga.sub.0.9 N layer 104 of 0.1 .mu.m in thickness, a Si-doped n type Al.sub.0.15 Ga.sub.0.85 N layer 105 of 0.4 .mu.m in thickness, a Si-doped n type gallium nitride layer 106 of 0.1 .mu.m in thickness, a multiple quantum well layer 107 wherein an In.sub.0.2 Ga.sub.0.8 N quantum well layer of 2.5 nm in thickness and an In.sub.0.05 Ga.sub.0.95 N barrier layer of 5 nm in thickness are repeated 26 times, a Mg-doped p type Al.sub.0.2 Ga.sub.0.8 N layer 108 of 20 nm in thickness, a Mg-doped p type gallium nitride layer 109 of 0.1 .mu.m in thickness, a Mg-doped p type Al.sub.0.15 Ga.sub.0.85 N layer 110 of 0.4 .mu.m in thickness and a Mg-doped p type gallium nitride layer 111 of 0.5 .mu.m in thickness, in this order. On the uppermost p type gallium nitride layer 111 is formed a p electrode 112 consisting of two layers of nickel (Ni) and gold (Au); on the n type gallium nitride layer 103 is formed an n type electrode 113 consisting of two layers of titanium (Ti) and aluminum (Al). Used as the cavity mirror surface of the laser is an etched surface obtained by reactive ion etching. Or, as described in Japanese Patent Application Kokai (Laid-Open) No. 64912/1996 regarding "Semiconductor Light-Emitting Device and Production Thereof", on the r- or m-face of a sapphire substrate is formed a gallium nitride type compound semiconductor layer; the layer is subjected to etching to expose the (0,0,0,1) face; and this face is used as the end surface of an optical waveguide. Thus, in conventional nitride semiconductor lasers, the cavity mirror surface is formed by etching or by cleavage.
In these laser devices, however, the threshold current density is high; the life of room-temperature continuous oscillation is short; and no practical applicability is attained.
The threshold current density of semiconductor laser device depends greatly on the density of states at around the valence band top of the active layer semiconductor. A nitride semiconductor, having a very small spin-orbit splitting energy, has three bands at around the valence band top and has a large density of states, which is one reason for the high threshold current density of the nitride semiconductor laser.
Further, in conventional nitride semiconductor lasers, nitride compound semiconductors different in chemical composition are used in the active layer. Therefore, biaxial strain is inevitably introduced into the active layer owing to the difference in lattice constant. This strain gives rise to an internal electric field in a direction perpendicular to the substrate surface, owing to a piezoelectric effect. As a result, electrons and holes are separated spatially in the active layer; the overlapping of the wave functions is decreased; and the efficiency of recombination and light emission is reduced.
The above matters are presumed to be the reasons for high threshold current density in conventional nitride semiconductor lasers.
In addition, the cavity mirror surface of prior art formed by subjecting a gallium nitride type compound semiconductor layer to reactive ion etching has had poor flatness and consequently a large mirror loss. Also, the cavity mirror surface produced by cleavage has been unable to have good flatness because the sapphire substrate and the cleavage plane of nitride semiconductor are not well matched.